1. Meiosis and Sexual Reproduction
Chapter 7

2. 7.1 Meiosis
In some organisms haploid gametes join to form a diploid zygote
If gametes weren’t haploid imagine how the number of chromosomes would increase of time
Meiosis is the form of cell division that makes diploid cells haploid when forming gametes
There are 2 divisions in Meiosis – Meiosis I and Meiosis II
Before Meiosis begins DNA is replicated so cell has homologous chromosomes

3. Prophase I Chromosomes condense Nuclear envelope breaks down
Homologous chromosomes pair up
Crossing over occurs
Chromatids exchange

4. Metaphase I
Homologous pairs move to equator by spindle

5. Anaphase I Homologous pairs separate and pulled towards poles
Chromatids do not separate; still connected by centromeres

6. Telophase I Individual chromosomes gather at poles
Most organisms now have cytokinesis
2 new cells formed
Chromosomes do not replicate again

7. Prophase II
New spindle forms

8. Metaphase II
Chromosomes line up along equator and are still attached at centromeres

9. Anaphase II Centromeres divide
Chromatids separate; Now called chromosomes

10. Telophase II
Nuclear envelope forms around chromosome sets

11. Meiosis in Action

12. Meiosis and Genetic Variation
Three key ways meiosis creates new genetic variations
Independent Assortment
Crossing Over
Random Fertilization

13. Independent Assortment
Most organisms have more than 1 chromosome
In humans the gamete gets 1 chromosome from the 23 pairs
Which of the pair you get is chance
This random distribution is called Independent Assortment
Mathematically 223 = over 8.3 million combinations

14. Crossing Over
Portion of 1 homologous chromosome is broken off and exchanged with the corresponding portion of its homologous partner

15. Random Fertilization What egg and sperm meet is also completely random
Mathematically that means that the possible combinations of the sperm (about 8.3 million) can be multiplied by the combinations for the egg (about 8.3 million) giving 70 trillion possible combinations

16. Meiosis generates variation in a species very quickly
Pace of evolution is sped up due to variation
A + B = C, a new individual not identical to its parents

17. Gametogenesis – Formation of Gametes
Spermatogenesis – sperm production occurs in testes
Diploid cell increases in size and becomes immature germ cell
Meiosis I = 2 immature sperm cells
Meiosis II = 4 haploid cells
Develop tails and become sperm

18. Oogenesis – egg production occurs in ovaries
Diploid cell increases and becomes germ cell
Meiosis I = after cytokinesis cytoplasm is not equally divided
Smaller one called polar body
Meiosis II = larger cell divides again, again cytoplasm not equally divided
Forms another polar body
Polar bodies will die
Ovum (pl. = ova) is larger cell
More cytoplasm means more stored energy

19. 7.2 Sexual Reproduction Asexual Reproduction
Single parent passes on copies to offspring called “clone”
Identical to parent
Prokaryotes use binary fission

20. Many eukaryotes use asexual reproduction
Fission – parent into 2 equal sized offspring
Fragmentation
Body into several pieces
Missing parts grow back
Budding – new split off from old; may stay attached

21. Genetic Diversity Asexual reproduction is simplest method
In stable environment can produce many offspring in short time’
Don’t need to waste energy making gametes or find a partner
DNA varies very little
In changing environment may not be able to adapt

22. Sexual Reproduction
2 parents form reproductive cells with half the number of chromosomes
Diploid Mother makes haploid gamete
Diploid Father makes haploid gamete
Haploid gametes make diploid offspring different from parents

23. Evolution of Sexual Reproduction
May have come as a way for protists to repair DNA
Only diploid cells can repair
Protists form diploid cells under stress
Many enzymes that repair DNA are involved in meiosis

24. Sexual Life Cycle in Eukaryotes
Life Cycle – span from 1 generation to next
All sexually reproductive organisms have a diploid-haploid-diploid lifecycle
Only 3 Types of Sexual Life Cycles
Only difference is which phase becomes multicellular

25. Haploid – simplest and most common
Zygote is the only diploid cell and it undergoes meiosis immediately
Haploid cells in multicellular organism produce gametes through mitosis
Fusion – gametes fuse to form zygote and meiosis follows
Meiosis allows for damage repair
2 homologous chromosomes are lined up, special repair enzymes remove damaged sections of DNA and fill in gaps
Protists, Fungi, Algae

26. Haploid Life Cycle

27. Diploid Adults are diploid, inherit from 2 parents
Diploid cell then meiosis then haploid gamete
Gametes join during fertilization forming diploid zygote
Zygote divides by mitosis
Gametes are the only haploid cells

28. Diploid Life Cycle

29. Parthenogenesis New individual forms from unfertilized egg
Hypothesized that mothers chromosomes are copied instead of getting fathers chromosomes
Produces clones
Honeybees provide drones this way (haploid)
Whiptail lizards are all females
Doesn’t seem possible in mammals
Only mammal clones are twins from split egg

30. Alternation of Generations
Regularly alternates between haploid and diploid
Plants – diploid phase called sporophyte produces spores
Spore forming cells produce haploid spore that can form adult without another cell
Gametophyte is haploid phase
Produces gametes by mitosis
Gametes fuse and produce diploid phase
Diploid – Haploid – Diploid – Haploid – etc…
Moss – haploid spores develop at top of sporophyte stalk